This invention is concerned with a system for the measurement of linear and angular positions of one object relative to another, comprising a first inductive system that consists of a first transmitting coil and a first detector; a second inductive system that consists of a second coil and a second detector, wherein the said first transmitting coil has conductive elements arranged in a geometry such that displacement along one axis will change the magnetic flux through a selected area in a predetermined way, whereas the flux through the said area will remain essentially unchanged when the displacement is perpendicular to the said axis. Two or more sets of transmitter coils and detectors are used to sense displacement in several dimensions.
An electromagnetic system for the measurement of relative positions is a transducer that will convert a relative physical position and angular rotation between two bodies to a signal that can be interpreted as their relative position and rotation. Transducers for linear and angular position are applied in a multitude of technical and mechanical devices. In the following this will be referred to as a position measurement system.
A position measurement system is concerned with two objects which are movable relative to each other and for which the relative linear and or angular positions shall be determined. Each of the two objects will have components that can generate and/or detect an electromagnetic field. By generating an electromagnetic field at one object that has an predetermined spatial variation, detection at the other object will make it possible to record pairs of related values of electromagnetic flux and the relative position of the two objects. Detection of the magnetic flux thus makes it possible to determine said position between the two objects. The accuracy with which this can be accomplished depends on how well this relation can be established. As a general observation, the transmitted field could be generated in close proximity to the place of detection to avoid disturbances from other electromagnetic fields, including the Geomagnetic field.
Applications of position measurement systems include measurement of rotation about one or more axes and/or measurement of linear displacement in one or more dimensions. Some position measurement systems are generally applicable, others are dedicated to the measurement of eg rotation about one axis or linear displacement in one dimension.
In connection with measurement of more advanced patterns of relative motion, ie relative position and/or rotation in more than one degree of freedom, a trade-off must usually be made where accuracy is significantly reduced to obtain measurement in more than one degree of freedom. This is caused by mechanical limitations to the layout of transmitter and detector elements for measurement in two or three dimensional structures. The result is that a high accuracy can usually not be achieved.
U.S. Pat. No 5,523,683 describes an induction sensor to measure the displacement of one body relative to another using electromagnetic induction. The paper describes a scaler and a slider, each made as hollow cylinders on each of which a coil is constructed as a set of conductors. The induction sensor measures linear displacement along said cylinder, with the coils generating a field that is essentially varying along the axis of the cylinder, with the exception of a stray component from feed and return conductors axially on the cylinder. The transmitter and receiver coils are made as geometrically equivalent in a pattern to make it insensitive to any other motion than the desired linear displacement.
This induction sensor is thus capable of measuring linear displacement only and is insensitive to scaler and slider being rotated at an angle relative to each other.
GB patent application No 2,095,840 describes a system for the determination of position information based on magnetic induction. This system comprises a permanent magnet and magneto-sensitive semiconductor devices located around the permanent magnet.
This system has the drawback that the magnetic field is constant over time and thus rather sensitive to other magnetic fields, including the geomagnetic field and fields from magnetised objects. The geometry of this system is not made to compensate for such disturbances.
GB patent application Nom 2,197,078 describes a system for the determination of relative positions comprising a transmitter and a receiver. The transmitter comprises three coils arranged along orthogonal axes. The coils are driven pairwise by an alternating current, resulting in a field that rotates about the axis of the second coil. This field is detected using the receiver which can be a coil. The induced voltage in the coil can then be related to the position of the receiver relative to the transmitter. The receiver can comprise three orthogonal coils.
This system is not always useful since the achievable accuracy depends on the distribution of conductive material in proximity to receiver and transmitter. Furthermore, the geometry of the electromagnetic field is relatively complex which hinders a very accurate determination of position.
It is thus a difficulty with known electromagnetic position measurement systems to determine the position of an object in more than one degree of freedom in that the mechanical construction causes the generated electromagnetic field to have a complexity that in practice hinders the detection of the field such that the detected signals can be used for precise determination of relative position. It is a further obstacle that existing solutions do not make it possible to determine the achievable accuracy through simple geometry of the mechanical construction of the sensing device.
One purpose of the present invention is therefore to provide a position measurement system which can establish an electromagnetic field that can be precisely detected and uniformly in at least two degrees of freedom and with a desired accuracy.
This is achieved by constructing the aforementioned position measurement system such that the second transmitting coil comprises electrical conductors arranged in an essentially parallel way, which extend in a direction perpendicular to the axis in which a displacement is desired to be sensed, and distributed along the axis such that a second electromagnetic component can be established perpendicular to the said axis. The conductive elements are arranged in a geometry such that displacement along the said axis will change the magnetic flux through a selected area in a predetermined way, whereas the flux through said area will remain essentially unchanged when the displacement is perpendicular to the axis. Two or more sets of transmitter coils and detectors are used to sense displacement in two or three dimensions.
Conductor elements can be distributed along the said axis and a desired accuracy can be obtained using commonplace design. The result is a position measurement system that can establish a well-defined electromagnetic field, that can be easily detected and has a spatial variation along two or more orthogonal axes.
In a preferred embodiment the first transmitter coil comprises a second conductor element which is essentially parallel with the first conductor element. This enables an increase of the transmitted field strength.
In a specifically preferred embodiment, the first and second conductor elements have sufficient separation for current through the conductors generates (to generate) an electromagnetic field that can be detected with the first detector. This makes it possible to generate a well defined, spatially varying electromagnetic field in a desired region around the object.
A very compact geometry can be obtained when the first transmitting coil is ring-shaped.
In a particular form, the conductor elements in the second transmitter coil are connected to make a conductive pattern around a mid-line. The pattern can then be made according to a desired geometry of the detector such that a position dependent signal can be generated with desired properties.
In a particular form, the second transmitter coil is given a Meander pattern with the mid-line as axis of symmetry. A Meander coil has a periodically repeated pattern that can be repeated along its longitudinal axis and thus give a uniform longitudinal detectability along the entire coil. The return current is led through a conductor at the said mid-line, thus shaping the electromagnetic components as desired along as well as perpendicular to the longitudinal direction.
In another form, the first detector has a geometry that overlaps the first transmitting coil perpendicular to the longitudinal direction, such that a signal is generated that is sensitive to displacement perpendicular to the longitudinal direction while the signal is insensitive to displacement along the longitudinal axis. The detector can thus detect a position displacement perpendicular to the longitudinal axis, while the detector is also displaced along the longitudinal axis.
In a particular embodiment, the first detector comprises a first and a second detector coil that are overlapping with half a coil-width. This makes it possible to generate two redundant position signals, of which one detects a maximum when the other detects a minimum in flux. This can be used to achieve fault-tolerance of the sensor and also to enhance the detection accuracy. In addition, this makes detection insensitive to distance between the transmitter and detector coils.
In a particular embodiment, the second detector has a geometry that overlaps the second transmitter coil in a direction perpendicular to the longitudinal axis to enable generation of a position signal that is a function of the longitudinal displacement, but insensitive to displacement in the perpendicular direction.
In a further embodiment, the second detector comprises a third and a fourth detector coil which are overlapping with half a coil-width. Again, this makes it possible to obtain two position signals to yield enhanced accuracy of detection and ability to make a fault-tolerant construction. In addition, this makes detection insensitive to distance between the transmitter and detector coils.
In a further embodiment, the first transmitter coil is driven by a first electrical signal, the second by a second electrical signal, such that the electromagnetic fields generated by the two transmitter coils can be distinguished from each other. This is used for both discrimination in the detectors and for elimination of electromagnetic disturbances.
In a further embodiment, the first and second transmitter-coils are mounted on a surface. This makes the coil construction very compact in a direction perpendicular to the said surface.
In a form where the said surface has the shape of a cylinder or a section thereof, the first and/or the second detector is mounted on a double-curved, eg a spherical surface. This construction allows to maintain an essentially constant distance between transmitter and detector coils when turning or displacing the objects relative to each other. Although not required for the proper function, this will reduce effects of disturbing magnetic fields.